Obtaining accurate measurements of two port devices with a vector network analyzer (“VNA”) uses a two-port calibration to characterize the systematic errors of the instrument and test set. Calibration is typically performed using a number of calibration devices of pre-determined characteristics, such as a short, an open, a termination of known impedance, and one or more through lines of known electrical lengths, or an electronic calibration (“E-cal”) device, which is a programmable device that selectively provides a sequence of calibration standards. These devices are commonly known as “calibration standards.”
In many cases, a customer needs to alter the VNA or test set while measuring a device under test (“DUT”). One example of this is changing the value of an attenuator to optimize the measurement of both high and low power (either input or output) conditions of the DUT. Unfortunately, such a change in the test environment invalidates the two-port S-parameter calibration made with the VNA/test set in its previous state.
One technique for addressing this problem is to re-calibrate the test system (i.e. the VNA and test set) with the calibration standards after the test system has been configured to its new state. However, this requires disconnecting the DUT, connection of the calibration standards, and execution of a complete 2-port S-parameter calibration, requiring the measurement of multiple calibration standards, or multiple states of an E-cal device. While this approach produces good results, it is inconvenient, time consuming, and disrupts the test sequence flow of the DUT.
Another technique is to calibrate all desired states of the test system prior to measuring the DUT. This approach is time consuming, even if the customer knows in advance which states the test system needs to be calibrated for. A customer does not always know which test system states will be used when testing a DUT. If the customer does not know which test system states need to be calibrated for, a complete calibration of all possible states is usually done. This is very time consuming because each state requires execution of a complete 2-port S-parameter calibration, requiring the measurement of multiple calibration standards, or multiple states of an electronic calibration device.
Sometimes, approximations of the effects of a state change of the test system are used to avoid some portions of the time-consuming calibrations. Unfortunately, such approximations result in increased measurement uncertainty, and hence less accurate measurements.
Therefore, an improved technique for measuring a DUT after a calibrated port has been modified that provides accurate network measurements is desirable.